514 ANATOMY OF VERTEBRATES. 



called blood-discs, added to the f liquor sanguinis/ move in single 

 file along the terminal capillaries of the circulating system and 

 here come into the requisite contact with the tissues for the in- 

 terchanges in question. One visible result of the giving and 

 taking through attracting and repelling forces, usually defined as 

 6 vital,' l is the change of colour which here takes place, viz., from 

 florid to modena, in the general system, and the reverse in the 

 respiratory one. Agreeably with this view of the function of the 

 blood-discs we find them, in relation to the grade of histological 

 development in the class, to be the most numerous and most 

 minute relatively to the bulk of the body, in the present : in other 

 words, the collective circulating surface effecting organic inter- 

 change is greatest in the blood of Mammals. 



The blood-discs are squeezed in the narrowest tract of the 

 capillaries, and by their elasticity resume their shape in the wider 

 part: they are not constantly separated by plasma from the 

 capillary wall, and the thickness of that wall is very inferior to 

 that of the membrane which experiments have shown to allow of 

 endosmotic transit of matters. The mammalian blood-corpuscle, 

 as a general rule, is a circular disc ; and, instead of being swollen 

 in the centre by a nuclear part, is there thinner ; the disc is conse- 

 quently slightly biconcave : it consists of the albuminoid coloured 

 matter, insoluble in serum, called hematosine, the particles of 

 which have aggregated, according to their formifying forces, into 

 the discoid shape. The colour of the individual blood-disc is 

 yellow ; lighter in the middle where it is thinnest, deepening to a 

 red tint only when light is reflected from a thickness resulting 

 from an aggregate of many discs : the quantity of the disc- 

 substance similarly affects transmitted light. 



The average diameter of the human blood-disc is 3 q th of an 

 inch (vol. i, fig. 8, a). I early availed myself of the menagerie 

 of the London Zoological Society to test the characters of size 

 in the Mammalian class, and communicated the two extremes, ob- 

 served, e. g., in the Elephant and Pygmy Musk (ib. b), with some 

 other instances from different orders, including Marsupials and 

 Monotremes, so far as to determine the class-characteristic afforded 



1 • Tous les faits les mieux constates me semblent montrer que les globules du sang 

 ne sont pas de simples concretions inertes de matiere animale resultant d'une sorte de 

 precipitation ou de coagulation sphero'idale ; que ce sont au contraire des parties 

 vivantes;' ccxxxix. p. 80. Nevertheless if, as Acherson thought he observed, 

 (cxxxxm") the white or granular globules should be a result of reaction of oil-like 

 particles on proteine-matters in plasma, their manifestation of forces, though called 

 * vital,' would not be valid against an observed mode of ' spontaneous generation' or 

 1 formifaction ' of such globules. 



